Temperature sensors or generally thermometers have been widely applied in numerous fields such as measurements, instrumentation, control systems, etc. A temperature sensor circuit includes two bipolar junction transistors (BJTs) through which electric currents flow. Most temperature sensors are conventional sensors such as thermistors or platinum resistors that require separate readout circuitry. Recent development in temperature sensors includes sensors that output readily interpretable temperature readings in a digital format as well as the advent of smart temperature sensors that combine a temperature sensor with interface electronics on a single chip. Smart temperature sensors manufactured with the standard, low-cost CMOS (complementary metal-oxide-semiconductor) technology have their own limitations such as limited operating ranges (e.g., from −55-degree to 125-degree Celsius), relatively low accuracy compared to conventional temperature sensors due to process variations (e.g., within-die, from-die-to-die, cross-substrate, or cross-tools process variations) during the manufacturing of the smart temperature sensors.
Various improvements have been developed to improve the accuracy of smart temperature sensors by, for example, employing the one-point trimming techniques to trim transistor's emitter area and/or its bias current with a sigma-delta digital-to-analog converter and/or by adding a programmable PTAT (proportional to absolute temperature) voltage to certain transistors in smart temperature sensors to compensate for the spread in the nominal value of a transistor's saturation current and the spread of the bias current. In addition, the bandgap voltage from a typical bandgap voltage reference circuit in smart temperature sensors manufactured with the CMOS technologies exhibit second order effects and thus often require two-point trimming techniques to compensate for process variations. These one-point trimming techniques, two-point trimming techniques, or the addition of a programmable PTAT voltage are not only complex and often, if not always, require a larger silicon area for implementation. The requirement of a larger area on silicon offsets or even negates the low-cost benefit of manufacturing smart temperature sensors with the standard CMOS technologies.
Therefore, there exists a need for a CMOS-based current reference circuit that is independent of or at least insensitive to process variations and produces digital readout with improved accuracy yet without separate readout circuitry. The CMOS-based current reference circuit produces digital readouts with improved accuracy without trimming although the adoption of trimming techniques in some embodiments may further improve the accuracy of the digital readouts.